1. Field of the Invention
The present invention relates to a solid electrolyte capacitor and a method for manufacturing the same, and more specifically to a solid electrolyte capacitor comprising a solid electrolyte formed of a conducting polymer material and a method for manufacturing the same.
2. Description of Related Art
A solid electrolyte capacitor has such a construction that an anode is formed of a porous shaped member of a valve metal such as a tantalum or aluminum, a dielectric material layer constituted of a metal oxide film formed on a surface of the anode, and a cathode having its portion formed of a solid electrolyte layer formed on the metal oxide film. Since the solid electrolyte layer has a function of electrically connecting between the whole surface of the dielectric material formed on an inner surface of pores in the porous shaped member and an external cathode terminal for connecting with an external device, the solid electrolyte layer is preferably formed of a material having a high conductivity. In addition, the solid electrolyte layer is also required to have a function of healing an electric short-circuiting attributable to a defect in the dielectric film. As a result, a metal having a high conductivity but having no function of healing the dielectric film cannot be used as the solid electrolyte, and manganese dioxide has been used which is converted into an insulator because of heat generated by a short-circuiting current.
Furthermore, when the solid electrolyte capacitor is mounted together with other circuit components on a mounting board such as a printed circuit board, the solid electrolyte capacitor is exposed to a heat as high as 240.degree. C. to 260.degree. C. although the heat is temporary. In view of this problem, since the manganese dioxide has a satisfactory resistance to heat of not less than 240.degree. C., the manganese dioxide has been widely used.
As seen from the above, the material used as the solid electrolyte of the solid electrolyte capacitor are required to fulfill three conditions: (1) high electric conductivity; (2) the function of healing the dielectric material; and (3) resistance to heat of not less than 240.degree. C.
Here, the manganese dioxide widely used in the prior art as the solid electrolyte has a satisfactory characteristics in connection with the function of healing the dielectric material and the resistance to a high heat at the time of mounting. However, the conductivity of about 0.1 S/cm of the manganese dioxide cannot be necessarily said to be satisfactory as the solid electrolyte of the solid electrolyte capacitor. Under this circumstance, recently, there is energetically advanced the development of a solid electrolyte capacitor comprising a solid electrolyte formed of conducting polymer material such as polypyrrole, polythiophene, polyaniline, which has electric conductivity as high as 10 S/cm to 100 S/cm and which fulfills the three conditions required for the solid electrolyte of the solid electrolyte capacitor.
In general, the solid electrolyte capacitor utilizing the above mentioned conducting polymer material has some problems, which will be described hereinafter. First, it is necessary to form a conducting polymer layer on an inner surface of the porous shaped member without leaving a surface portion which is not covered with the conducting polymer material. Secondly, the conducting polymer material must have a satisfactory resistance to a heat of less than 240.degree. C. at the time of mounting. Thirdly, the conducting polymer material never lowers the electric conductivity at a high temperature which is different from the temporary high heat of less than 240.degree. C. exposed when the capacitor is mounted onto the printed circuit board, and which continues for a relative long time and reaches a relatively high temperature on the degree of 120.degree. C. to 150.degree. C. under an actual working environment in which a circuit having the mounted capacitor is in operation. Fourthly, the conducting polymer layer must be formed to have a film thickness of less than a predetermined value in a good film quality, so that the underlying dielectric oxide film is in no way damaged by a stress generated in a thermal expansion and shrinkage of a housing resin member (thermal stress).
Of the above mentioned four conditions, the second condition, namely, the resistance to heat at the time of mounting is a nature inherent to the polymer, determined by a chemical bonding condition of the conducting polymer. On the other hand, the third condition, namely, the resistance to temperature in the actual working environment is determined by the degree of so a called "de-doping" in which dopants giving electric conductivity to polymer are lost from the polymer. Since these second and third conditions are different from each other in a determining factor, the resistance to a high heat at the time of mounting, directed to the second condition, will be called a "resistance to high heat" hereinafter, and the resistance to a high temperature in the actual working environment will be called a "resistance to high temperature" hereinafter.
In the above mentioned problems of the solid electrolyte capacitor, Japanese Patent Application Pre-examination Publication No. JP-A-02-015611 and its corresponding U.S. Pat. No. 4,910,645 (the content of which is incorporated by reference in its entirety into this application) proposes a solid electrolyte capacitor comprising an electrolyte layer formed of polythiophene derivative which obtains electric conductivity by doping p-toluenesulfonic acid or methane-sulfonic acid. In this patent, for forming a layer of polythiophene derivative, an iron (III) compound (ferric compound) is used so that polythiophene derivative monomer is oxidatively polymerized by oxidative reaction from iron (III) (ferric ion) to iron (II) (ferrous ion).
The polythiophene derivative used in the electrolyte capacitor disclosed in the above referred patent has the "resistance to high heat" higher than other polymer such as polypyrrole derivative, namely, is more excellent in the second condition ("resistance to high heat") of the above mentioned four conditions. In addition, as regards the film quality, polypyrrole is in the form of powder, and therefore, poor in denseness, and to the contrary, the polythiophene derivative forms a film. In addition, the film quality of the polythiophene derivative is the most dense as compared with other polymer including polyaniline, other than polypyrrole. Therefore, the electrolyte capacitor comprising the solid electrolyte formed of polythiophene derivative can sufficiently resist to a stress generated in a thermal expansion and shrinkage of a housing resin member (thermal stress). In addition, since the polymer layer is dense, the film thickness required to have a necessary degree of "resistance to thermal stress" can be made small. In other words, since the electric resistance of the solid electrolyte layer can be made smaller than the case that other conducting polymer materials are used, a high frequency characteristics of the capacitor can be elevated.
As seen from the above, the solid electrolyte capacitor disclosed in the above referred patent is excellent in the "resistance to high heat" of the second condition and in the "resistance to thermal stress" in the fourth condition, of the four conditions required for this type of capacitor.
Recently, however, electronic instruments are rapidly increasing its field of application with advancement in performance of electronic instruments and in compactness and light weight of electronic instruments. Under this inclination, electric parts incorporated in an electronic circuit are strongly required to have an elevated performance including an improved characteristics and an elevated reliability (degree of resistance to deterioration in performance caused by an irreversible change with lapse of time under a constant condition), although working environment and condition have a tendency to degrade. For the solid electrolyte capacitor, it is strongly required to elevate the "resistance to high temperature" (namely, to prevent deterioration of the electric resistance of the conducting polymer material in use under a high temperature) and also to elevate the "resistance to thermal stress". Furthermore, the solid electrolyte capacitor having an excellent characteristics and high reliability should be preferably inexpensive, and therefore, is preferred to be manufactured in accordance with a process, which is high in efficiency of chemical oxidative polymerization at the time of forming the conducting polymer layer, and which can shorten the manufacturing process and can reduce the amount of raw material used.